Kernel: Switch singletons to use new Singleton class

MemoryManager cannot use the Singleton class because
MemoryManager::initialize is called before the global constructors
are run. That caused the Singleton to be re-initialized, causing
it to create another MemoryManager instance.

Fixes #3226

Kernel/CommandLine.cpp

@@ -38,6 +38,7 @@ const CommandLine& kernel_command_line()
 
 void CommandLine::initialize(const String& string)
 {
+    ASSERT(!s_the);
     s_the = new CommandLine(string);
 }